Wondering where to start when comparing performance ratings? The National Fenestration Ratings Council (NFRC) tests window products for thermal performance. These ratings are tested in a laboratory, are completely objective, and test the total unit performance. Beware of window contractors and sales people that quote ratings that are taken at the center of the glass, or from frame materials.

U factor (also known as U value) is the most referenced metric of performance when it comes to replacement windows, and for good reason. The total unit U factor tests the entire, finished window assembly for thermal performance at a 5mph wind speed. Essentially, this rating tells you how well the unit will insulate, and specifically measures the rate at which heat can pass through, without regard to air infiltration. U factor is actually the inverse of the more commonly referenced “R factor” when it pertains to other building materials, however U factor is specifically used for fenestration (glass). To convert, […]